The American Rescue Plan Act of 2021 enables states to lengthen eligibility for pregnancy-related Medicaid coverage from the current sixty days after birth to up to one year, a time when mothers remain at elevated pregnancy-related health risk. Using linked birth records, income, and all-payer claims data for Medicaid-paid births in Colorado during the period 2014-19, we compared continuity of coverage during one year postpartum among people eligible for low-income adult Medicaid (with incomes of 138 percent of the federal poverty level or lower) versus those ineligible for Medicaid by any pathway (with incomes of 139 percent of poverty or higher). We found that retention of Medicaid coverage as a low-income adult was associated with 1.5 additional months of postpartum insurance enrollment and a 12-percentage-point increase in the probability of continuous insurance coverage during the first year after birth. Our findings suggest that states that adopt the American Rescue Plan Act option to provide eligibility for pregnancy-related benefits for a full year after birth are likely to improve continuity of postpartum insurance coverage.